1. Field of the Invention
The present invention relates to communications systems and, in particular, to a digital frequency shift keying (FSK) demodulator circuit which receives a hard-limited FSK input signal and recovers the modulating baseband signal at 300 baud or less.
2. Discussion of the Prior Art
The basic function of a communications system is to transmit information over a communication channel between an information source and a destination which are physically separated from one another.
Communications channels designed to handle voice transmissions (i.e., a telephone network) have inherent characteristics which make it difficult for them to transmit digital data or bit streams. Therefore, to permit the transmission of digital data over voice channels, it is necessary to convert the digital data at the transmission point to a form which is compatible with the voice channel. This is done by utilizing the digital data to modulate a carrier waveform which is within the voice frequency range, transmitting the modulated signal and then demodulating the signal at the receiver to separate the transmitted data from the modulated carrier waveform.
Data transmission systems that operate at low data transmission rates, i.e. 1200 baud or less, typically use a modulation technique known as frequency shift keying (FSK). According to this technique, the two binary states are represented by two different frequencies and are detected by using two frequency tuned sections, one tuned to each of the two frequencies. The demodulated signals are then integrated over the duration of a bit and a binary decision is made based upon the result.
Digital transmission using FSK modulation has a number of advantages. First, the implementation is similar in complexity to a conventional AM transmission system. Second, since the received signals can be amplified and limited at the receiver, a simple limiting amplifier can be used, whereas an AM system requires sophisticated automatic gain control in order to operate over a wide level range. Furthermore, FSK modulation can show a 3 or 4 dB improvement over AM systems in most noise environments, particularly at distortion threshold; that is, as the frequency shift becomes greater, the advantage of FSK over AM improves in a noisy environment. A further advantage of FSK modulation is its immunity to the affects of non-selected level variations, even when these variations are extremely rapid. Thus, FSK modulation is used almost exclusively in worldwide high frequency radio transmissions where rapid fades are a common occurrence. In the United States, FSK modulation has nearly universal application for the transmission of data at 1200 baud and below.
A common disadvantage of conventional analog FSK demodulator circuits is that they are sensitive to circuit parameter variations and are not suitable for large scale integration.
Conventional digital FSK demodulator signal processing requires A/D converters, high power consumption and very high system toggling frequencies which results in heavy expenditures on RFI shieldings.